When operating in hot gas environments, joining a ceramic thermal insulating material to a metallic structure requires a good control of the stress level in the ceramic thermal insulating material in order to avoid premature failure of the ceramic material. In order to achieve this, it is interesting to design the joint of the ceramic material and metallic material for the highest possible temperature, in order to minimize the required thickness of the ceramic thermal insulating material, such that the thermal stresses in such ceramic material part are reduced, as they are directly related to the temperature gradient on said part. The benefit of a high temperature joint on the thermal gradient in the ceramic layer is counterbalanced by a higher stress level at the joint due to the difference of thermal expansion coefficients of the ceramic and of the metallic substrate. Besides, the higher the temperature of the metallic material during operation, the higher the oxidation rate of the metallic material will be; therefore, the metallic material composing the joint needs to have a high oxidation resistance.
It is known in the state of the art to join a ceramic thermal insulating material to a metallic structure by means of brazing of the ceramic part to the metallic part, using active brazing, reactive air brazing or metallization of the ceramic material. However, all these known solutions are limited in temperature capability, either due to the low melting point of the active braze alloys that are used (based on Ag or Au) when active or reactive air brazing is used, or due to the poor oxidation resistance of the metal used when metallization of the ceramic material is done, this metal used for metallization being typically Mo or Mn.
Another possibility known in the art is to join the ceramic material and the metallic material by means of mechanical joining: this solution allows the selection of the materials to be used specifically for their functional properties with minimum constraints on materials compatibility. However, when a mechanical joining solution is used, the problem is that stress concentration occurs at the joining location, which leads to a local risk of cracking of the ceramic material, which can propagate catastrophically through the whole ceramic material, leading to its premature failure.
Other solutions known in the art are, for example, fitting the ceramic in a metallic clamping system, having the problems as described for the mechanical joining stated above, or using high temperature cements, presenting the problem of a brittle joining layer with limited mechanical properties subjected to high stress levels, leading to possible local cracking that can propagate and cause a premature failure of the ceramic material.
The present invention is directed towards providing a joining configuration that solves the above-mentioned problems in the prior art.